This invention relates in general to engine exhaust systems and, more particularly, to an improved exhaust system for four-stroke reciprocating internal combustion engines.
A great deal of effort has been expended over the years in improving the efficiency and power output of four-stroke piston engines, particularly for racing applications. Improvements in engine components and "super tuning" engines have resulted in racing engines producing well over one horsepower per cubic inch displacement. These engines have been perfected to the point where proportionally smaller improvements are becoming more difficult to achieve.
Tuned exhaust systems have been found to produce significant increases in power, especially in high speed racing engines.
One phenomenon employed in tuning an exhaust system has to do with the motions of sound pressure waves inside the exhaust system. These sound pressure waves having no mass travel at the speed of sound and consist of condensations and rarefactions in the gas. As a positive pressure wave resulting from the piston pushing gases out of the cylinder moves down to the end of the exhaust pipe and surges into the atmosphere, a negative wave (or slight vacuum) is created and moves back up the pipe. Ideally, the length of the exhaust system is such that a negative wave will arrive back at the engine exhaust valve just before the valve closes, so that the slight vacuum of the negative wave can aid in extracting the last of the combustion products from the cylinder.
The length of the exhaust pipe which provides the optimum scavenging from the negative wave can be determined from the empirical formula: L=120 V/N, where "L" is the length of the pipe in inches from the valve head to the outer end, "N" is the desired peaking speed in revolutions per minute and "V" is the speed of the wave in the exhaust gases in feet per second. At usual temperatures and atmospheric pressure, "V" is about 1700 feet per second. In a racing engine, "N" will be about 7000 rpm. This produces an optimum pipe length of about 29 inches. Unfortunately, at idle speed, about 1000 rpm, optimum pipe length becomes about 204 inches. Thus, with the 29 inch high speed exhaust pipe, as engine speed drops from the optimum high speed, low speed reversion occurs causing the waves to become out of phase with the engine and instead of extracting gases from the cylinder, will compress gases back into the cylinder causing the engine to stutter and lose power. This is a significant problem with engines which must operate efficiently over a wide range of engine speeds.
Attempts have been made to improve the exhaust system efficiency by exhausting gases through a short nozzle into a relatively large volume which communicates with the final exhaust pipe. A typical such system, although for use in two-stroke engines, is disclosed in U.S. Pat. No. 2,168,528. While this system may have some advantages in a two-stroke engine, it does not appear suitable for a high efficiency four-stroke engine. The large volume into which the short exhaust nozzle empties will act similarly to the atmosphere and produce an immediate negative wave. As detailed above, this negative wave will, at best, arrive back at the cylinder at the proper time only over a very limited range of piston speeds.
Thus, there is a continuing need for improved exhaust systems for high efficiency four-stroke engines to make the maximum use of the "negative pressure wave" effect to optimize scavenging of exhaust gases at a wide range of engine operating speeds.